Caption: Computer generated space-filling model of nine actin monomers from an F-actin helix, a polymer of the protein actin. For clarity each actin monomer (the repeated polymer unit) is shown in a different colour. Each amino-acid residue is represented by a sphere of radius 2.7 angstroms. In muscle cells, actin forms part of the thin filament, which cyclically interacts with the thick myosin filament to produce a mutual sliding that is the basis of muscle contraction. The green spheres are amino-acid residues that cross-link to the myosin in the actomyosin complex. The structure of the F-actin filament was determined using a technique called X-ray fibre diffraction.